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Biomedical subjects

R Lechler

Publications and source records attributed to R Lechler.

At least 55 records · Page 3Linked to original sources
PubMed1994

Anergic T cells as suppressor cells in vitro.

T cell-mediated suppression is an established phenomenon, but its underlying mechanisms are obscure. An in vitro system was used to test the possibility that anergic T cells can act as specific suppressor cells. Anergic human T cells caused inhibition of antigen-specific and allospecific T cell proliferation. In order for the inhibition to occur, the anergic T cells had to be specific for the same antigen-presenting cells (APCs) as the T cells that were suppressed. The mechanism of this suppression appears to be competition for the APC surface and for locally produced interleukin-2.

Antigen-Presenting Cells↗
PubMed1994

Failure of correlation between B7 expression and activation of interleukin-2-secreting T cells.

It is well established that triggering interleukin-2 (IL-2) secretion by helper T cells requires the T cell to receive at least two discrete signals. One signal is transduced by the CD3 complex, usually as the result of T cell receptor (TcR) occupancy, the second, or co-stimulatory, signal involves a non-cognate interaction between cell surface accessory molecules on the antigen-presenting cell (APC) and the T cell. A molecular interaction that has been implicated in the provision of co-stimulatory signals is that between B7/BB1 on the APC and its ligands, CD28 and CTL-A4 on the T cell. We have studied the ability of HLA-class II antigen-positive human T cells and a population of DR1-expressing transfected human fibroblasts to stimulate a proliferative response by human T cell clones, and by freshly isolated peripheral blood T cells. Despite their high levels of B7 expression, the T cell clones, were unable to induce proliferation or IL-2 secretion by DR-restricted, antigen-specific T cells. In contrast, the DR1-expressing transfectants, that were B7 negative, induced a strong proliferative response. When these two populations of DR-expressing cells were used to stimulate a primary alloresponse the results were reversed, in that the T cell clones induced a strong alloresponse but the transfected fibroblasts induced no proliferation. These results suggest that the expression of B7 may be necessary for costimulation of unprimed T cells, but not of established T cell clones. Furthermore the data show that the expression of B7 by an APC does not necessarily lead to IL-2 production or protection from the induction of tolerance. The mechanisms responsible for the inability of these T cells to provide full activation signals when used as APC is currently under investigation.

Antigen-Presenting Cells↗
PubMed1994

Different regions of the N-terminal domains of HLA-DR1 influence recognition of individual peptide-DR1 complexes.

The contributions of individual amino acids in the polymorphic beta chain and the conserved alpha chain of HLA-DR1 to influenza HA-specific DR1-restricted and anti-DR1 allospecific T-cell recognition were analyzed. The genes encoding HLA-DR1 were subjected to site-directed mutagenesis in order to introduce single amino acid substitutions at 12 positions in the beta 1 domain and 11 positions in the alpha 1 domain. The beta 1-domain substitutions were all at polymorphic positions and introduced residues that are found in DR4 alleles. The amino acids introduced into the DR alpha 1 domain were based on the sequences of other human and mouse class II alpha chains. The responses of 12 DR1-restricted T-cell clones specific for two peptides of HA and seven anti-DR1 allospecific clones were studied. Substitutions at positions that point up from and into the peptide-binding site in the third variable region of the beta 1-domain alpha-helix caused substantial reduction in the responses of all of the clones. Substitutions at multiple positions in the beta 1-domain floor and in the alpha 1 domain influenced the anti-DR1 responses of the alloreactive and of the HA100-115-specific T-cell clones. In contrast, very few changes outside of the beta 1 domain third variable region affected the responses of the HA306-324-specific DR1-restricted T-cell clones. These results suggest that a surprisingly limited region of the HLA-DR1 molecule is critically involved in T-cell recognition of HA306-324 by DR1-restricted T cells. However, the susceptibility of the HA100-115-specific and the anti-DR1 allospecific T-cell clones to substitutions at multiple positions in both N-terminal domains shows that the response to DR1-HA306-324 is unusual and may reflect the promiscuity with which this peptide binds to HLA-DR molecules.

Amino Acid Sequence↗
PubMed1993

Analysis of accessory signaling in human T-cell clones.

Human T cells, when activated, express small but detectable levels of MHC class II on their surface and as a result have the potential to present antigens in the context of MHC class II molecules. There are reports demonstrating MHC class II restricted antigen presentation by human T-cell clones. In this report, we show evidence for one such clone that can present peptide antigen to itself. This clone, HA1.7 ml, is clearly different in its accessory signaling requirements from the parental clone HA1.7, and shows a decrease in the surface level of CD54. We have analyzed the accessory signaling abilities of T cells by using HA1.7 ml as a responder population, and demonstrate that the accessory signaling potential of T-cell clones is sufficient to bring about activation of HA1.7 ml but not of HA1.7. This accessory signaling ability is, however, different from that of B cells in providing bystander accessory signals. T-cell APCs cannot provide bystander accessory signals for the proliferative response of HA1.7 ml, nor can they block the induction of tolerance in this subline, whereas bystander B cells can mediate both events to a limited extent. Thus, there are significant differences in the accessory signaling abilities of T cells as APCs as compared with classic APCs such as B cells, independent of their ability to generate peptide-MHC complexes. It also appears that although HA1.7 ml can be activated by self-presentation of antigen, it is still susceptible to the induction of tolerance.(ABSTRACT TRUNCATED AT 250 WORDS)

Antibodies, Monoclonal↗
PubMed1992

A rapid limiting dilution assay for measuring frequencies of alloreactive, interleukin-2-producing T cells in humans.

A limiting dilution analysis (LDA) has been established which measures the total numbers of alloreactive interleukin-2 (IL-2)-secreting T cells in human peripheral blood mononuclear cells (PBMC). A significant advantage over most previous LDA is that the assay may be completed in approximately 48 h since an IL-2-dependent 'indicator' cell line is used to reduce assay time. Results are reproducible and correlate with the degree of HLA class II antigenic disparity between responder and stimulator cells. Use of both PBMC and Epstein-Barr virus-transformed B lymphoblastoid cell lines (B-LCL) as stimulator cells permits estimation of the frequency of Epstein-Barr virus-specific T cells in different responder individuals. A modification of the assay may also be used to measure the frequencies of 'primed' alloreactive cells, i.e., those alloreactive cells which have previously encountered their specific stimulating alloantigen. Use of the assay in the clinical context of bone marrow and renal transplantation is discussed.

Animals↗
PubMed1992

Use of a murine T-cell hybridoma expressing human T-cell receptor alpha- and beta-gene products as a tool for the production of human T-cell receptor-specific monoclonal antibodies.

We describe the production of mouse monoclonal antibodies specific for the human TcR using as the immunogen transfected murine T-cell hybridoma cells coexpressing mouse CD3 with human Jurkat TcR alpha and beta chains. The shortage of monoclonal antibodies (mAbs) specific for the human TcR-V alpha and V beta families reflects the difficulties in their production by conventional methods using whole human T cells or purified soluble receptors as immunogens. As an alternative strategy to circumvent these difficulties, we have generated a transfected mouse T-cell line expressing a human (Jurkat) TcR alpha beta dimer in a complex with mouse CD3. The parental mouse T-cell line, TG40, is a cell surface TcR-negative, cytoplasmic CD3-positive variant of the mouse T-cell hybridoma 2B4. The human-TcR alpha beta expressing mouse transfectant was used to immunize mice with the same genetic background as the parent mouse T-cell line, and a human TcR-specific response was successfully achieved. MAb-producing hybridomas were generated by fusing spleen cells from the immunized mice with the mouse myeloma cell line NSO. Of 124 hybridoma supernatants screens, 72 showed reactivity to the human T-cell line Jurkat. Twenty-four of the hybridomas producing human (Jurkat) TcR-specific antibodies were cloned and screened for reactivity to Jurkat TcR. Several IgG2b and IgM mAbs specific for the Jurkat T cell line were selected on the basis of their ability to modulate surface CD3 expression on Jurkat cells. Most of the antibodies do not stain other TcR-expressing human T cell leukemia cell lines, implying specificity for the variable domains of the Jurkat TcR.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1992

Transfection of HLA-DR-expressing DAP.3 cells with a cDNA clone encoding the glycosyl phosphatidylinositol-linked form of lymphocyte function associated antigen-3: biochemical features and functional consequences.

Structural and functional aspects of the accessory molecule lymphocyte function associated antigen (LFA)-3 (CD58) have been examined following the transfection of DAP.3 and P815 cells with a cDNA clone encoding the glycosyl phosphatidylinositol (GPI)-linked form of human LFA-3. Despite earlier observations that DAP.3 cells are deficient in GPI anchoring LFA-3 was expressed efficiently on DAP.3, as well as on P815 cells. Immunoprecipitation of LFA-3 from 35S-labelled cells revealed that the molecule expressed on the DAP.3 cells had a molecular weight intermediate between the transmembrane and GPI-linked forms expressed by human B cells. This suggests that the DAP.3 cells have a default pathway whereby the RNA transcript which encodes the GPI-linked form of the molecule can also encode an integral membrane protein. Functionally, expression of LFA-3 by DAP.3 which had previously been transfected with the genes encoding HLA-DR1 led to a marked augmentation of the proliferative response of five out of eight anti-DR1 human T cell clones. This effect was not reproduced when DR1 and LFA-3 were expressed by separate populations of DAP.3 cells, suggesting that the ligands for CD2 and for the T cell's receptor must be expressed on the same cell membrane. Expression of human LFA-3 also led to a substantial increase in the proliferative response of human peripheral blood T cells to a DR alloantigen. Separation of T cells into CD45RO+ and CD45RO- populations revealed that the augmentation was more marked for the memory than the virgin population. The mechanisms responsible for these differences are discussed.

Animals↗
PubMed1991

Limited regions of the alpha 2-domain alpha-helix control anti-A2 allorecognition: an analysis using a panel of A2 mutants.

The regions of the HLA-A2 molecule controlling anti-A2 alloreactivity were explored using naturally occurring allelic variants of HLA-A, and a panel of transfectants expressing the products of A2.1 genes that had been mutated at multiple positions encoding residues in the alpha 2 domain alpha-helix. As a means of detecting distant conformational effects, these altered A2.1 molecules were also examined serologically. Amino acid substitutions at the carboxy-terminal end of the alpha 2 domain alpha-helix led to diminished staining with the monoclonal antibody (mAb) MA2.1. The epitope for this antibody has previously been mapped to the alpha 1 domain alpha-helix (residues 62-65). This suggests that interdomain contacts may cause conformational alteration, and that mutants can have distant, as well as local effects. Of the 24 positions where substitutions were made, only six led to loss of the anti-A2 alloresponse by the three clones and three lines that were tested. In addition, the mutations that altered the MA2.1 epitope, located on the alpha 1 domain alpha-helix, did not inhibit allorecognition. This suggests that a limited number of regions on the A2.1 molecule are responsible for allodeterminant expression. The most influential substitutions were those at positions 152, 154, 162, and 166. It is notable that three of these are predicted to be T-cell receptor (Tcr)-contacting residues, and one (152) to contribute to peptide binding. These results suggest that the specificity of alloreactive T cells is determined by exposed polymorphisms, directly contacted by the Tcr, and by concealed polymorphisms which influence peptide binding.

Cell Line↗
PubMed1991

Generation and characterization of an HLA-DR alpha-specific monoclonal antibody using L-cell transfectants expressing human and mouse class II major histocompatibility dimers.

Among the HLA-DR-specific monoclonal antibodies (mAbs) that have been characterized previously, there is a marked shortage of DR alpha chain-specific mAbs which stain unfixed cells. This may result from the high degree of sequence similarity between the alpha 1 domains of HLA-DR and H-2E leading to a state of cross-tolerance to DR alpha in H-2-expressing mice. BALB/b (E-negative) mice were immunized with DR1-transfected mouse L cells. The chain specificity of the resulting DR-specific mAb was determined using a panel of transfectants expressing hybrid mouse/human class II heterodimers. A DR alpha-specific mAb was generated which was capable of immunoprecipitating DR alpha beta dimers and inhibiting the anti-DR alloresponse of human T-cell clones. The present study demonstrates that, with the selection of a suitable recipient strain, transfectants can be useful in the generation and definition of chain-specific mouse mAbs.

Animals↗
PubMed1991

The relationship between MHC restricted and allospecific T cell recognition.

The existence in the mature T cell repertoire of a high precursor frequency of cells which recognise allogeneic MHC molecules appears to contradict the well-established dogma of positive selection for self MHC restriction. In order to explore the possibility that alloreactive cells are derived from a fraction of the repertoire that is not self-MHC-restricted, the contribution of in vivo-primed T cells to "primary" alloresponses was investigated. Peripheral blood T cells were separated into virgin and memory populations by sorting for low and high levels of LFA-3 expression, and their proliferative responses to MHC incompatible stimulator cells was quantitated. The results demonstrated that approximately half of a "primary" alloresponse is contributed by previously primed T cells that, by definition, must be self-MHC restricted. Furthermore it was possible to define the original MHC-restricted antigen specificity of two T cell clones raised against the allospecific HLA-DR1 from a DR4Dw4/DRw13DW19 responder. The emerging consensus view that anti-MHC alloreactive T cells, like antigen-specific T cells, are specific for MHC/peptide complexes, and have a parental self-MHC restriction, begs a structural explanation. Comparison of multiple DR beta 1 domain sequences reveals that DR molecules fall into groups that have extensive homology in the residues on the beta 1 domain alpha-helix that are predicted to point up towards the T cell receptor (histotopic), and thus to determine MHC restriction. Given that the DR alpha chain is invariant this creates the possibility that anti-DR allorecognition can mimic self-restricted recognition. Within these groups of histotopically similar DR products there are multiple differences in the peptide-binding residues that lie on the inner aspects of the alpha-helix or on the floor of the antigen-binding groove. As a consequence, it is predicted that a different array of endogenous peptides will be bound, due to determinant selection. Thus, allorecognition within these groups may result from the recognition of endogenous peptides that are bound by stimulator but not by responder MHC products, seen in a self-restricted manner. In combinations where histotopic similarity does not exist, allorecognition may be best explained by the chance occurrence of a receptor selected for intermediate affinity for thymically expressed MHC molecules having a higher affinity for an allogeneic histotope. Such a receptor would have been deleted in a thymus expressing the allospecificity, but would be perceived as "safe" in the absence of this MHC product.

Antigen-Antibody Reactions↗
PubMed1991

Hard graft? Future challenges in transplantation.

A small group of transplantation surgeons, immunologists and molecular biologists gathered in Vienna in early February to discuss the prospects for organ transplantation. Participants at the meeting were challenged with setting goals for transplantation research and with speculating on how this research might influence the practice of transplantation in the next two decades. Some goals were set, but the most vigorous discussion focused on the existing barriers that stand in the way of achieving these goals.

Antigens↗
PubMed1991

Structural aspects of allorecognition.

The phenomenon of T-cell allorecognition of foreign major histocompatibility molecules has been one of the more enigmatic aspects of T-cell immunology. The molecular basis for allorecognition is unfolding as a result of the application of major histocompatibility complex structure/function analyses in the light of current insights into the three-dimensional structure of major histocompatibility complex products.

Animals↗
PubMed1991

Stress-induced modulation of antigen-presenting cell function.

The effect of two means of inducing a stress response, heat and oxygen radicals, on the ability of an HLA-DR1 B-cell line to stimulate DR1-restricted and anti-DR1 auto- and alloreactive T-cell clones has been examined. Both forms of stress enhanced the ability of B cells to stimulate auto- and alloreactive T-cell clones and to present peptide to an influenza-virus specific T-cell clone. Furthermore, the ability of the B-cell line to present whole influenza virus was augmented by heat stress. The stress-induced enhancement of T-cell responses coincided with a modest increase in the cell-surface expression of major histocompatibility class II products. This was, however, insufficient to account for the observed functional effects. In contrast to these effects, presentation of whole antigen was inhibited by the oxygen radical intermediate, hydrogen peroxide (peroxide), in a dose-dependent manner. When analysed by SDS-PAGE, it was found that whilst overall protein synthesis decreased following both types of stress, increased synthesis of heat-shock proteins (HSP), and in particular the 70,000 MW HSP, was only evident following heat stress. The absence of an increase in the synthesis of HSP 70, in the antigen-presenting cells (APC) following the uptake of UV-treated influenza virus, however, implied that HSP 70 induction was not necessary for the presentation of whole antigen. The effects of peroxide stress appeared to be qualitatively different in several respects. First, peroxide treatment did not cause the induction of any stress proteins; second, peroxide abolished the presentation of whole antigen. In addition, heat stress of APC was unable to protect from the adverse effects of peroxide treatment, in that cells treated sequentially with heat, followed by peroxide, were unable to present whole influenza virus. In order to determine the stage of antigen presentation at which peroxide was causing inhibition, APC were treated at varying time-points after pulsing with antigen. The kinetics of the peroxide effect paralleled those of aldehyde fixation. Taking these results together it would appear that peroxide interferes with some aspects of the antigen-processing pathway.

Antigen-Presenting Cells↗